The present invention relates to a method and apparatus to detect and measure a static electric charge on an object. More specifically, the invention concerns a method and apparatus for the accurate detection and measurement at a safe distance of a static charge on an object wherein the method and apparatus are applicable for experimental and industrial applications.
The detection and the measurement of static charge is one of the oldest problems experienced in both experimental work and in industrial applications. Although the problem has existed for some time, there has been little improvement in methods and apparatus used for static charge detection and measurement. This is especially true for non-contact static charge measurement.
Heretofore, there have been three basic methods commonly used for static charge measurement.
One basic method comprises the use of an electrostatic voltmeter. The use of an electrostatic voltmeter to measure the static charge has a major drawback because the probe of the electrostatic voltmeter has to contact the charged object. The contact may cause a spark. A spark could ignite a combustible atmosphere, such as that which exists in a chemical refining environment. The drawbacks and basic impracticality of using an electrostatic voltmeter become apparent.
Another method comprises establishing a force balance between a known electrical field and the charged object. However, the electrical field can only be applied to measure small non-fixed objects. Such limitations make the force balance method useless in real world applications.
The third method uses field mill instruments. For non-contact charge measurement, the field mill instruments (generating voltmeters) can sense the charge at a distance. However, present generating voltmeters are not true remote sensing instruments.
In a field mill instrument, the electric field is sensed through the sensing plate by applying a mechanical chopping device, i.e. a segmented rotor which is connected to the ground, to rotate in the front of the sensor. The change of the sensing area will then induce an alternating current which one can measure. The problem associated with this type of device is the inaccuracy of the measurement.
Two reasons cause this inaccuracy. First, the chopping process generates significant electronic noise which is difficult to filter. Electronic noise interferes with accurate measurement of a static charge. Second, for low voltage electric fields, the guard plate and chopping rotor used in these devices distort the field so much that very little of the actual field will impress on the sensor, hence, measurement cannot be performed accurately.
Because of accuracy problems, the measurement, which has an accuracy of no better than 10.sup.4 V/m, can only be done at a very short distance. In general, the measuring distance for a field mill instrument is less than one inch. For an electric field lower than 10.sup.4 V/m, it is very difficult to obtain an accurate measurement of the static charge with a field mill instrument. One example of a field mill generator is in Schwab, A.J., High-Voltage Measurement Techniques, pp. 141-146, The MIT Press, Cambridge, Mass. (1972).
Although static discharge does little harm to the human body, it can be detrimental in many situations. In some cases, industrial environments have explosive atmospheres. For example, a spark induced by a highly charged surface may trigger a dangerous explosion in chemical plants or petroleum refining facilities.
Industrial settings may include sensitive equipment. This delicate electronic equipment may be easily damaged by a static discharge. Further, the problem of static discharge has become more acute because of the wide applications of computer chips. A static discharge may easily damage computer chips.
It therefore becomes apparent that the ability to accurately and safely detect the build-up of high voltage static charge can be used to great advantage in laboratory and industrial situations. The accurate and safe detection of static charge can reduce the possibility of explosion in hazardous environments. It can reduce the possibility of damage to sensitive equipment including computers equipment including computer chips.